1. Field of the Invention
This invention relates in general to a computer-implemented storage mechanism, and more particularly, to a storage mechanism for persistent objects in a database management system.
2. Description of Related Art
Databases are computerized information storage and retrieval systems. A Relational Database Management System (RDBMS) is a database management system (DBMS) which uses relational techniques for storing and retrieving data. Relational databases are organized into tables which consist of rows and columns of data. The rows are formally called tuples. A database will typically have many tables and each table will typically have multiple tuples and multiple columns. The tables are typically stored on random access storage devices (RASD) such as magnetic or optical disk drives for semipermanent storage.
RDBMS software using a Structured Query Language (SQL) interface is well known in the art. The SQL interface has evolved into a standard language for RDBMS software and has been adopted as such by both the American National Standards Institute (ANSI) and the International Standards Organization (ISO). The SQI, interface allows users to formulate relational operations on the tables either interactively, in batch files, or embedded in host languages, such as C and COBOL. SQL allows the user to manipulate the data.
The definitions for SQL provide that a RDBMS should respond to a particular query with a particular set of data given a specified database content, but the method that the RDBMS uses to actually find the required information in the tables on the disk drives is left up to the RDBMS. Typically, there will be more than one method that can be used by the RDBMS to access the required data. The RDBMS will optimize the method used to find the data requested in a query in order to minimize the computer time used and, therefore, the cost of doing the query.
Traditionally, a RDBMS stored simple data, such as numeric and text data. In a traditional RDBMS, the underlying storage management has been optimized for simple data. More specifically, the size of a record is limited by the size of a data page, which is a fixed number (e.g., 4K) defined by a computer developer. This restriction in turn poses a limitation on the length of columns of a table. To alleviate such a restriction, most computer developers today support a new built-in data type for storing large objects (LOBs). Large objects, such as image data, typically take up a great deal of storage space. However, the access time for LOB object data is significantly slower than that of simple data types. This occurs because the LOB object data is accessed through a LOB descriptor stored in a record buffer and requires an extra input/output (I/O) access of a database. In many applications, where the average size of an object is typically much smaller than the largest size of the object, using a LOB descriptor is the only way to store object data, but storage comes at the cost of performance. This problem also applies to structured object types, such as abstract data types, arrays, lists, sets, etc. There is a need in the art for an improved method of storing persistent data.